Oxidative stress is linked with cardiovascular and other major diseases such as cancer, acute inflammation, Parkinson's, and Alzheimer's. In 2007, an estimated $430 billion was spent to treat 80 million people suffering from some type of cardiovascular disease. Antioxidant agents play a vital role in the defense against free radicals and oxidant molecules via radical scavenging, metal ion chelation, and co-antioxidant action. The balance between oxidative species and antioxidant systems defines the oxidative stress of a living system. The oxidative stress level determines when a biological system is at high risk for various diseases.
Atherosclerosis in particular has been related to oxidative processes. High levels of oxidant molecules, or low levels of antioxidants are linked to high oxidative stress that can trigger atherosclerosis. The “oxidation theory” of atherosclerosis considers free radicals to be precursors in development of the disease. Vitamin E, especially α-tocopherol, is a natural antioxidant present in the lipophilic phase of every cell. Among the lipophilic antioxidants, α-tocopherol is one of the most potent. Other important lipophilic antioxidants are coenzyme Q10 and the carotenoids.
Low-density lipoprotein (LDL) comprises a core of cholesterol and triglycerides, surrounded by a layer of fatty acids, cholesteryl esters, apolipoproteins, and phospholipids. The outer layer is more hydrophilic, while the core is more hydrophobic. Oxidation of LDLs has been associated with the development of atherosclerosis. The ox-LDL (i.e. the oxidized form of LDL) stimulates the host response and the formation of foam cells.
Lipophilic antioxidants inhibit oxidation of the lipid fraction of LDLs, but apolipoproteins are not protected from oxidation even in the presence of lipophilic antioxidants.
Hydrophilic antioxidants can protect hydrophilic molecules such as proteins, DNA, and carbohydrates from oxidation, and they can also regulate some oxidative molecules for signaling purposes. Examples of hydrophilic antioxidants include vitamin C and carnosine. Others include uric acid, and certain flavonoids, micronutrients, peptides, and proteins.
The dipeptide carnosine (β-alanyl-L-histidine) is known to have antioxidant properties, and also to suppress protein glycation and crosslinking. Carnosine has also been suggested as an agent for controlling secondary problems in diabetes.
Several synthetic antioxidants have been reported—e.g., butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), propyl gallate; and vitamin E derivatives, including Trolox™, 3-oxa-chromanol derivatives, brominated alpha-tocopherol methanol-dimer, raxofelast, vitamin EC, and probucol. Several synthetic antioxidant molecules reported in the literature use the OH group of α-tocopherol to attach other molecules to α-tocopherol, or to modify the chromanol ring. These prior modifications have, however, decreased the specificity of the α-TTP interaction for α-tocopherol.
It has been suggested that α-tocopherol can act alternatively either as a pro-oxidant or as an anti-oxidant, and that this switch in behavior is induced by the level of oxidative stress. At low oxidative stress, α-tocopherol acts as a pro-oxidant, promoting lipid peroxidation. At high oxidative stress, α-tocopherol acts as an anti-oxidant. Co-antioxidants such as coenzyme Q-10 may play a role in promoting the anti-oxidant behavior of α-tocopherol. However, the pro-oxidant action of vitamin E has not yet been proven in mammals.
In vivo and in vitro assays have shown that Vitamin E is effective in protecting LDL from oxidation. However, clinical trials involving Vitamin E in patients with coronary artery disease or atherosclerosis have given conflicting results.
There is an unfilled need for new anti-oxidant compounds for the treatment and prevention of oxidative-stress-associated diseases.
Vitamin E
Vitamin E is a lipophilic vitamin with both antioxidant properties and non-antioxidant activities. The Vitamin E family includes four tocopherols (α, β, γ, δ) and four tocotrienols (α, β, γ, δ), the difference being that the tocotrienols have unsaturated phytyl chains. The differences between the α, β, γ, and δ compounds lie in the number and positions of the methyl groups on the aromatic ring of the tocopherol or tocotrienol.
The main antioxidant functions of vitamin E are to trap peroxyl radicals, and to break chain reactions of lipid peroxidation. Vitamin E also quenches superoxide anions (O2.−), singlet oxygen (1O2), and hydroxyl radicals (.OH); and vitamin E inhibits the reaction of nitric oxide (NO) with O2.−.
The antioxidant activity of tocopherols is related to the methylation pattern and to the number of methyl groups on the phenolic ring in the order: α>β>γ>δ.
Supplementation with vitamin E has not shown conclusive results in the prevention or control of diseases such as cardiovascular disease, cancer, or atherosclerosis, although clinical trials have suggested that some patients with pre-existing cardiovascular disease can benefit from supplementation with α-tocopherol (up to 800 IU/day or 537 mg/day).
Carnosine (β-Alanyl-L-Histidine)
Carnosine is a hydrophilic dipeptide (β-alanyl-L-histidine) that is synthesized exclusively by mammals. It is found in high concentrations in muscle and brain tissue, both of which can experience high oxidative stress. Human skeletal muscle levels of carnosine range from 2 to 20 mM. In the brain, carnosine is nonuniformly distributed with higher levels (up to 5 mM) in the olfactory epithelium and the bulbs. Carnosine and its analogs homocarnosine and anserine are degraded by the enzyme carnosinase, which is actually a group of intra- and extracellular dipeptidases.
The imidazole moiety of carnosine has been associated with the dipeptide's antioxidant properties. It has been suggested that the protons on the nitrogen ring and on the methylene carbon adjacent to the imidazole ring are required for antioxidant activity.
Carnosine can protect proteins from oxidation. Carnosine may also have beneficial effects in diabetes, and in certain neurodegenerative diseases that are associated with high levels of zinc, such as Alzheimer's disease.
Tatsunori et al., Japanese Patent Application Publication 2008019188A (English abstract, 2008) discloses a carnosine derivative and its use in medicines, skin cosmetics, and the like for treating or preventing diseases caused by damage due to active oxygen species, for preventing aging, and the like.
S. L. Stvolinsky et al., “Biological Activity of Novel Synthetic Derivatives of Carnosine,” Cell Mol Neurobiol (2010) 30:395-404 (published online 2 Oct. 2009) discloses two derivatives of carnosine, (S)-trolox-L-carnosine and (R)-trolox-L-carnosine, having antioxidant properties.
See also: Y. S. Wang et al., “Synthesis of Alpha-Tocohexaenol (Alpha-T6) a Fluorescent, Oxidatively Sensitive Polyene Analogue of Alpha-Tocopherol,” Bioorganic & Medicinal Chemistry (2010) 18:777-786; A. Hosomi et al., “Affinity for Alpha-Tocopherol Transfer Protein as a Determinant of the Biological Activities of Vitamin E Analogs,” FEBS Lett (1997) 409:105-108; M. Schuelke et al., “Urinary Alpha-Tocopherol Metabolites in Alpha-Tocopherol Transfer Protein-Deficient Patients,” J Lipid Res (2000) 41:1543-1551; and S. W. Leonard et al., “Incorporation of Deuterated RRR— or All-Rac-Alpha-Tocopherol in Plasma and Tissues of Alpha-Tocopherol Transfer Protein-Null Mice,” American Journal of Clinical Nutrition (2002) 75:555-560.